How to write a simple OBJ exporter with Python?(With Normals and UV)
Okay, thanks to lemon for this answer:


  If I remember well, vertex normals are not at the "face level" but at the "vertex level" in OBJ. So you should export them the same way you did for 'v', outside of the polygons loop 


I added this code in the vertex loop:

f.write( 'vn %f %f %f\n' % (vert.normal.x,vert.normal.y,vert.normal.z))

And used the same index for both the vertices and normals in the face definitions and it imports correctly!